In physical cosmology, the Planck epoch (or Planck era) is the earliest period of time in the history of the universe, from zero to approximately 10−43 seconds (Planck time). It is believed that, due to the extraordinarily small scale of the universe at the time, quantum effects of gravity dominated physical interactions. During this period, approximately 13.79 billion years ago, gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable. As it expanded and cooled, the familiar manifestations of the fundamental forces arose through a process known as symmetry breaking.

Modern cosmology now suggests that the Planck epoch may have inaugurated a period of unification, known as the grand unification epoch, and that symmetry breaking then quickly led to the era of cosmic inflation, the Inflationary epoch, during which the universe greatly expanded in scale over a very short period of time.[1]

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As there presently exists no widely accepted framework for how to combine quantum mechanics with relativistic gravity, science is not currently able to make predictions about events occurring over intervals shorter than the Planck time or distances shorter than one Planck length, the distance light travels in one Planck time—about 1.616 × 10−35 meters. Without an understanding of quantum gravity, a theory unifying quantum mechanics and relativistic gravity, the physics of the Planck epoch are unclear, and the exact manner in which the fundamental forces were unified, and how they came to be separate entities, is still poorly understood. Three of the four forces have been successfully integrated in a common framework, but gravity remains problematic. If quantum effects are ignored, the universe starts from a singularity with an infinite density. This conclusion could change when quantum gravity is taken into account. String theory and loop quantum gravity are leading candidates for a theory of unification, which have yielded meaningful insights already,[citation needed] but work in noncommutative geometry and other fields also holds promise for our understanding of the very beginning.

Experimental data casting light on this cosmological epoch has been scant or non-existent until now, but recent results from the WMAP probe have allowed scientists to test hypotheses about the universe's first trillionth of a second (although the cosmic microwave background radiation observed by WMAP originated when the universe was already several hundred thousand years old). Although this interval is still orders of magnitude longer than the Planck time, other experiments currently coming online including the Planck Surveyor probe, promise to push back our 'cosmic clock' further to reveal quite a bit more about the very first moments of our universe's history, hopefully giving us some insight into the Planck epoch itself. Data from particle accelerators provides meaningful insight into the early universe as well. Experiments with the Relativistic Heavy Ion Collider have allowed physicists to determine that the quark–gluon plasma (an early phase of matter) behaved more like a liquid than a gas, and the Large Hadron Collider at CERN will probe still earlier phases of matter, but no accelerator (current or planned) will be capable of probing the Planck scale directly.